1. Field of the Invention
The present invention relates in general to a bicycle, and more particularly to a bicycle having a folding frame suitable for reducing its total length and rendering a crankshaft located on or behind a vertical center line of a saddle to prevent undesirable fatigue of a rider.
2. Description of the Prior Art
Conventionally, the bicycles are classified into two types, that is, bicycles for practical use such as a commuter's bicycle, a cargo bicycle and the like, and special bicycles such as a racing bicycle, a mountain bicycle and the like.
In a known bicycle irrespective of its type, it is required to shorten the distance between its front wheel and its rear wheel in order to achieve a desired mobility, a desired high speed and desired lightness. However, it is known to those skilled in the art that there is a limit in shortening the distance between the front wheel and the rear wheel of the bicycle irrespective of its type due to its intrinsic structure as described hereinbelow.
With reference to FIG. 1, there is shown a known bicycle in a side view. The bicycle includes a front wheel rotating shaft 21, a chain wheel rotating shaft 22, a rear wheel rotating shaft 23, a saddle 24 and a handle bar 25. As shown in this drawing, the distance Z between the front wheel rotating shaft 21 and the rear wheel rotating shaft 23 is equal to the sum of the distance A between the front wheel rotating shaft 21 and the chain wheel rotating shaft 22 and the distance B between the chain wheel rotating shaft 22 and the rear wheel rotating shaft 23.
It has been noted that there is a limit in shortening the distance A between the front wheel rotating shaft 21 and the chain wheel rotating shaft 22. That is, a minimum safety space A1 should be defined between the front end of the rotating circle of the pair of crank pedals rotatably mounted on individual cranks and the rear end of the front wheel in order to prevent interference between the rotating crank pedals and the front wheel which is to be freely turned about its turning shaft rightwards and leftwards to change the running course of the bicycle, thus to limit shortening of the distance A.
On the other hand, the limit in shortening the distance B between the chain wheel rotating shaft 22 and the rear wheel rotating shaft 23 is caused by a minimum safety space B1 which should be defined between the front end of the rear wheel and the chain wheel rotating shaft 22. From the above description, it will be apparent to those skilled in the art that the presence of the chain wheel rotating shaft 22 causes the limit in shortening the distance Z between the front wheel rotating shaft 21 and the rear wheel rotating shaft 23. Hence, when the chain wheel rotating shaft 22 is removed from the position between the front and rear wheels, it is possible to desirably reduce, preferably to minimize the distance Z between the front wheel rotating shaft 21 and the rear wheel rotating shaft 23.
In addition, the saddle of the known bicycle, on which the waist and the buttocks of a rider functioning as a central point of the rider's body are placed, is located behind the vertical center line of the chain wheel rotating shaft 22 in question and this causes unnaturalness of the rider's posture. That is, the chain wheel rotating shaft 22 is regarded as the most important element of the bicycle to which all rider's power is devoted since it is a central point of the pedal rotating operation generating the driving force of the bicycle. However, since the saddle is placed at the rear of the chain wheel rotating shaft 22 and the handle bar is placed at the far front of the shaft 22, the rider should zealously rotate, in order to run the bicycle, the pair of pedals about the shaft 22 as greatly bending his body about the waist and the buttocks laid on the saddle. Such a sitting posture is regarded as an unnatural posture for exercise carried out by the lower part of the body when considering the fact that a human being walks in an erect posture. Such a sitting posture, therefore, prevents the rider from using his maximum quantity of motion and troubles prevention of undesirable fatigue imposed on the rider.
In order to overcome the aforementioned problem, it is required to place the chain wheel rotating shaft 22 on or behind the vertical center line of the saddle, however, this location of the shaft 22 also causes the rear wheel rotating shaft 23 to be shifted to a position which is a long away behind the present position shown in the drawing. The distance Z between the front wheel rotating shaft 21 and the rear wheel rotating shaft 23 or the total length of the bicycle should be thus remarkably increased and this opposes to the recent trend of improved mobility, of high speed and of lightness of bicycles, especially of modern sports or leisure bicycles. One approach to the location of the chain wheel rotating shaft behind the vertical center line of the saddle without lengthening the distance between the front wheel rotating shaft and the rear wheel rotating shaft is to reduce the diameter of the rear wheel to be less than that of the front wheel. However, such a reduction of the diameter of the rear wheel can not effectively solve the above problem.